A Planetary System Around Our Nearest Star is Emerging

Scientists are getting closer to discovering Earth-like planets close to Earth

The Alpha Centauri stellar system, being our closest stars, has been a long-time destiny of science fiction stories and movies as the first future human venture into planets around other stars. Unfortunately, many previous scientific efforts to detect planets in these stars came up just plain empty handed. Science fiction has not been supported by science, until now.

Today, a team of European scientists announces the discovery of the first planet around a star in the Alpha Centauri system. The planet is as massive as Earth, only 13% more massive, although too hot for life. This new result opens the possibility that there might be other Earth-size planets in the Alpha Centauri system, including some potentially habitable.

The Alpha Centauri stellar system consists of three stars about 4.3 light years away. The largest two, Alpha Centauri A and B, are separated by 17.6 times the Sun-Earth distance in a highly elliptical orbit (almost the distance between the Sun and Uranus) and a third red dwarf star, Proxima Centauri, is much farther away and many times not considered part of the system.

Alpha Centauri A and B are also very similar to our Sun with respect to their size, brightness, activity, temperature, and age. Alpha Centauri A is 20% larger than the Sun while Alpha Centauri B is 10% smaller. Proxima Centauri is slightly closer to us but it is a very dim red star nearly one-tenth the size of the Sun.

The new planet, Alpha Centauri Bb (Alf Cen B b for short), orbits the secondary star Alpha Centauri B with a period of just 3.2 days at 0.04 the Earth-Sun distance from the star. At this short distance, the planet receives about 310 times more light from its star than Earth receives from the Sun, making its surface very hot, nearly 870°C, almost three times as hot as Mercury.

Out of the over 800 exoplanets (extrasolar planets) that have been detected and confirmed in the last twenty years, Alpha Centauri Bb is now, not only the nearest one to Earth, but also the closest to an Earth-mass around a Sun-like star. There are a few other smaller exoplanets, but around other type of stars. Previously, since the year 2000, Epsilon Eridani b was considered the closest exoplanet, 10.4 light years away.

All known potential habitable exoplanets so far are superterrans (aka Super-Earths), which are two to ten times more massive than Earth. Scientists do not know if these superterrans are less habitable or even more habitable than Earth. There are not planets in such massives ranges in our Solar System to compare to; our eight planets are either more or less massive.

One of the main goals of exoplanet studies is to find terran planets in the Habitable Zone, those between 0.5 to 2 Earth masses and orbiting at the right distance from their parent star to support liquid water. These are worlds that we can understand and relate to better than the more exotic nature of superterrans.

“This result represents a major step towards the detection of a twin Earth in the immediate vicinity of the Sun. We live in exciting times!” said Xavier Dumusque from the Geneva Observatory, Switzerland and Centro de Astrofisica da Universidade do Porto, Portugal, lead author of the research.

This research was presented in a paper An Earth mass planet orbiting Alpha Centauri B, that appeared online in the journal Nature on October 17, 2012.

Figure 1. Comparison of the size and orbit of Alpha Centauri B around Alpha Centauri A. Alpha Centauri B nearly takes 80 years to go around Alpha Centauri A in a highly eccentric orbit. The orbit of the planet around Alpha Centauri B is to small to be shown at this scale. Note: this images assumes a semi-major axis of 17.57 AU for the orbit of Alpha Centauri B, but other similar estimates has been suggested. CREDIT: PHL @ UPR Arecibo.

Figure 2. Relative size of the Alpha Centauri stars and its planet. The planet Alpha Centaury B b is also shown to scale similar to Earth-size. The Sun for comparison. CREDIT: PHL @ UPR Arecibo

Figure 3. Star appears big in photos because of the scattering of light by the telescope's optics as well as in the CCD sensors. Atmospheric effects also distort them for ground observatories. In reality, they are small dots even smaller than the apparent size of Pluto, thus challenging our astronomical instruments limits. The orbit of Alpha Centauri B b, shown as an ellipse around Alpha Centauri B, is almost as large as Pluto as seem from Earth. Unfortunately, astronomers do not have the capability yet to separate and detect the light from Alpha Centauri Bb from the star. Exoplanets are detected because they affect their parent star light by partially blocking it (transit method) or Doppler effects (radial velocity method). CREDIT: PHL @ UPR Arecibo, picture of Pluto and Charon by the Gemini Observatory/NSF/NASA/AURA.

Figure 4. Measured and estimated properties of Alpha Centauri B b represented in this SER computer generated image as a highly tectonically active planet due to tidal heating. Its Earth Similarity Index (ESI) of 0.27, well below the 0.8 to be considered an Earth-like planet, shows that although it has a similar mass as Earth this is a very different world. CREDIT: PHL @ UPR Arecibo.

Figure 5. Comparison of the Stellar Habitable Zones (SHZ) of Alpha Centauri A and B. Planets need to have the right size and orbit within the green areas to be considered potentially habitable, or allow liquid water in their surface. The new planet, Alpha Centauri B b, is too close to the star at 0.04 AU corresponding to a period of 3.2 days. Habitable planets around Alpha Centauri B need to be between 0.5 to 1.4 AU from the star or have a period between 136 to 636 days. Earth orbit, 1 AU, is shown as a dotted circle for reference (it looks smaller in the first circle but this is just and opticall illusion, check with a ruler). CREDIT: PHL @ UPR Arecibo.